Putting Indent in plugins array before specific plugins breaks the plugin

[Mgsy]

Originally reported here - https://github.com/ckeditor/ckeditor5-build-decoupled-document/issues/24.

It looks like position of some plugins in the editor's plugins array has an impact on Indent functionality and indentCommand.isEnabled is always false.

After adding Indent to the build and keeping this plugins order:

ClassicEditor.builtinPlugins = [
    Essentials,
    Indent,
    IndentBlock,
    Paragraph
]

Indent plugin won't work. But, if we put Paragraph before Indent, it will work:

ClassicEditor.builtinPlugins = [
    Essentials,
    Paragraph,
    Indent,
    IndentBlock
]

The same thing occurs if Paragraph and Heading plugins are loaded, but in this case, the position of Paragraph plugin doesn't have any impact on Indent in favour of Heading.

Will work:

ClassicEditor.builtinPlugins = [
	Essentials,
	Heading,
	Indent,
	IndentBlock,
	Paragraph
];

Won't work:

ClassicEditor.builtinPlugins = [
	Essentials,
	Indent,
	IndentBlock,
	Heading,
	Paragraph
];

cc @jodator @Reinmar @mlewand

[jodator]

We should move extending schema to afterInit() to fix this:

https://github.com/ckeditor/ckeditor5-indent/blob/172d1978b1561ee8bd9f68ea7c69944af653b8b7/src/indentblock.js#L58-L62

right now it is done in init() and other plugins defines schema in init() method. It's a bug as we should remember (that was me here) to extend/rely on schema checks to be done in afterInit() calls.

[jodator]

It's a 15 min change TBH so I'm adding this to current iteration.